Multigrade lubricating oils typically are identified by two numbers such as 10W30, 5W30 etc. The first number in the multigrade designation is associated with a maximum low temperature (e.g. -20.degree. C.) viscosity requirement for that multigrade oil as measured typically by a cold cranking simulator (CCS) under high shear, while the second number in the multigrade designation is associated with a minimum high temperature (e.g. 100.degree. C.) viscosity requirement. Thus, each particular multigrade oil must simultaneously meet both strict low and high temperature viscosity requirements in order to qualify for a given multigrade oil designation. Such requirements are set e.g., by ASTM specifications. By "low temperature" as used herein is meant temperatures of typically from about -30.degree. to about -5.degree. C. By "high temperature" as used herein is meant temperatures of typically at least about 100.degree. C.
The minimum high temperature viscosity requirement, e.g. at 100.degree. C., is intended to prevent the oil from thinning out too much during engine operation which can lead to excessive wear and increased oil consumption. The maximum low temperature viscosity requirement is intended to facilitate engine starting in cold weather and to ensure pumpability, i.e., the cold oil should readily flow or slump into the well for the oil pump, otherwise the engine can be damaged due to insufficient lubrication.
In formulating an oil which efficiently meets both low and high temperature viscosity requirements, the formulator may use a single oil of desired viscosity or a blend of two lubricating oils of different viscosities, in conjunction with manipulating the identity and amount of additives that must be present to achieve the overall target properties of a particular multigrade oil including its viscosity requirements.
The natural viscosity characteristic of a lubricating oil is typically expressed by the neutral number of the oil (e.g. S150N) with a higher neutral number being associated with a higher natural viscosity at a given temperature. In some instances the formulator will find it desirable to blend oils of two different neutral numbers, and hence viscosities, to achieve an oil having a viscosity intermediate between the viscosity of the components of the oil blend. Thus, the neutral number designation provides the formulator with a simple way to achieve a desired base oil of predictable viscosity. Unfortunately, merely blending oils of different viscosity characteristics does not enable the formulator to meet the low and high temperature viscosity requirements of multigrade oils. The formulator's primary tool for achieving this goal is an additive conventionally referred to as a viscosity index improver (i.e., V.I. improver).
The V.I. improver is conventionally an oil-soluble long chain polymer. The large size of these polymers enables them to significantly increase Kinematic viscosities of base oils even at low concentrations. However, because solutions of high polymers are non-Newtonian they tend to give lower viscosities than expected in a high shear environment due to the alignment of the polymer. Consequently, V.I. improvers impact (i.e., increase) the low temperature viscosities (i.e. CCS viscosity) of the base oil to a lesser extent than they do the high temperature viscosities. Accordingly, constraints are placed on the amount of V.I. improver which a formulator can employ for a given oil blend in order to meet the low and high temperature viscosity requirements of a target multigrade oil.
The aforesaid viscosity requirements for a multigrade oil can therefore be viewed as being increasingly antagonistic at increasingly higher levels of V.I. improver. For example, if a large quantity of V. I. improver is used in order to obtain high viscosity at high temperatures, the oil may now exceed the low temperature requirement. In another example, the formulator may be able to readily meet the requirement for a 10W30 oil but not a 5W30 oil, with a particular ad-pack (additive package) and base oil. Under these circumstances the formulator may attempt to lower the viscosity of the base oil, such as by increasing the proportion of low viscosity oil in a blend, to compensate for the low temperature viscosity increase induced by the V.I. improver, in order to meet the desired low and high temperature viscosity requirements. However, increasing the proportion of low viscosity oils in a blend can in turn lead to a new set of limitations on the formulator, as lower viscosity base oils are considerably less desirable in diesel engine use than the heavier, more viscous oils.
Further complicating the formulator's task is the effect that dispersant additives can have on the viscosity characteristics of multigrade oils. Dispersants are frequently present in quality oils such as multigrade oils, together with the V.I. improver. The primary function of a dispersant is to maintain oil insolubles, resulting from oxidation during use, in suspension in the oil thus preventing sludge flocculation and precipitation. Consequently, the amount of dispersant employed is dictated and controlled by the effectiveness of the material for achieving its dispersant function. A typical U.S. Service Station commercial oil contains from three to four times as much dispersant as V.I. improver (as measured by the respective dispersant and V.I. improver active ingredients). In addition to dispersancy, conventional dispersants can also increase the low and high temperature viscosity characteristics of a base oil simply by virtue of their polymeric nature. In contrast to the V.I. improver, the dispersant molecule is much smaller. Consequently, the dispersant is much less shear sensitive, thereby contributing more to the low temperature CCS viscosity (relative to its contribution to the high temperature viscosity of the base oil) than a V.I. improver. Moreover, the smaller dispersant molecule contributes much less to the high temperature viscosity of the base oil than the V.I. improver. Thus, the magnitude of the low temperature viscosity increase induced by the dispersant can exceed the low temperature viscosity increase induced by the V.I. improver without the benefit of a proportionately greater increase in high temperature viscosity as obtained from a V.I. improver. Consequently, as the dispersant induced low temperature viscosity increase causes the low temperature viscosity of the oil to approach the maximum low temperature viscosity limit, the more difficult it is to introduce a sufficient amount of V.I. improver effective to meet the high temperature viscosity requirement and still meet the low temperature viscosity requirement. The formulator is thereby once again forced to shift to the undesirable expedient of using higher proportions of low viscosity oil to permit addition of the requisite amount of V.I. improver without exceeding the low temperature viscosity limit.
In accordance with the present invention, dispersants are provided which have been found to possess inherent characteristics such that they contribute considerably less to low temperature viscosity increases than dispersants of the prior art while achieving similar or greater high temperature viscosity increases. Moreover, as the concentration of dispersant in the base oil is increased, this beneficial low temperature viscosity effect becomes increasingly more pronounced relative to conventional dispersants. This advantage is especially significant for high quality heavy duty diesel oils which typically require high concentrations of dispersant additive. Furthermore, these improved viscosity properties facilitate the use of V.I. improvers in forming multigrade oils spanning a wider viscosity requirement range, such as 5W30 oils, due to the overall effect of lower viscosity increase at low temperatures while maintaining the desired viscosity at high temperatures as compared to the other dispersants. More significantly, these viscometric properties also permit the use of higher viscosity base stocks with attendant advantages in engine performance. Furthermore, the utilization of the dispersant additives of the instant invention allows a reduction in the amount of V.I. improvers required.
The materials of this invention are thus an improvement over conventional dispersants because of their effectiveness as dispersants coupled with enhanced low temperature viscometric properties. These materials are particularly useful with V.I. improvers in formulating multigrade oils.
U.S. Pat. No. 4,548,724 discloses dispersant additives for use in lubricating oils formed by the reaction of polyacids and polyisobutenyl succinimide of a polyamine. The polyacids are organic polycarboxylic acids represented by the formula EQU R* (COOH).sub.x
wherein x is an integer of 3-6, preferably 3, and R* is a x valent hydrocarbon radical. This patent teaches that because of the fact that each reactant contains a plurality of reacting groups, the resulting product may not be a single compound but will undoubtedly include compounds containing an intricate network of products formed as a result of different amine groups of one molecule of succinimide bonding with a carboxyl group on different molecules of acid and different carboxyl groups of one molecule of acid bonding with an amine group on different molecules of succinimide. However, in the instant invention because the anhydride moiety is relatively more reactive with the secondary amino moiety than the carboxyl moiety, the formation of the compounds of the instant invention proceeds more rapidly and at less extreme reaction conditions than that using the polyacid. The use of a polyanhydride offers the further advantage that after reaction of the polyanhydride such as a bis-anhydride there is left a dicarboxylic acid moiety for still further reaction with the reactive amino groups.